1. Field of the Invention
The present invention relates generally to the field of electrical relays and breakers and more particularly to AC/DC intelligent solid state relay/breaker devices with current/temperature sensing, time-delay overloads and over-current/over-temperature protection.
2. Description of Related Art
Electromechanical relays have been used in the past in a wide-variety of power control and electrical applications. These mechanical devices which are generally constructed with a coil and contacts have demonstrated considerable reliability although they suffer from problems associated with having moving parts. In addition, mechanical relays are subject to arching and sparking. In applications, where it is required to switch a high DC voltage, the cost of a mechanical relay grows very rapidly. Also, the switching of the coil leads to voltage spikes (a fly-back voltage.) A lot of power is required to control the coil, and in high power relays, the coil can consume tens of watts. Material fatigue can shorten the life of a mechanical relay, and reliability can suffer due to shock and vibration.
These types of mechanical issues are major concerns when the relay is used in harsh environments. For example, many vehicles, such as submarines, cars, tractor/trailers, heavy vehicles and aircraft use a wide variety of relays in their systems. These relays are subject to constant vibrations introduced by the operation of the vehicles. Furthermore, many times relays built with mechanical contacts are exposed to environmental corrosive substances (liquid gases and the like) that lead to breakdown.
In addition to mechanical problems, electromechanical relays create abrupt “on” and “off” transitions thus introducing large transitional spikes. A large current surge many times welds contacts creating undesirable “shorted-on” malfunctions. In addition, sparking caused by connecting and disconnecting contacts can ignite surrounding gases and start fires.
Nowadays, some power devices are built with internal protection using a field-effect transistor with an integrated current and temperature sensing. This allows building a self-protective power device when there are only low-voltage field-effect transistors available. Some designs place a small-value resistor inserted in series with the load to measure a bypass current. This helps in some cases, but the extra generated heat makes that method unpopular. In addition, a current sense resistor adds to the overall resistance of the channel and thus reduces the efficiency of the device.
What is badly needed is an intelligent solid state Relay/Breaker that is integrated into a single package which exhibits a low output terminal resistance equal to the Rds(on) of the MOSFETs that is equipped with internal protection and control circuits to make the device rugged, efficient and compact for military, industrial or automotive loads in harsh AC and DC environments.